Congenic AKR.Fv-1(b) mice express about 10 to 50-fold less ecotropic virus in the thymus than AKR mice, and do not develop thymic lymphomas at 6 to 12 months of age like their AKR counterparts. To study whether the Fv-1(b) effect is mediated by thymocytes or thymic stroma, reciprocal subcutaneous thymus graft and bone marrow chimeras were constructed between AKR/Cum (Fv-1n, Thyl.1) and AKR.Fv-1(b) (Thyl.2) mice. Thyl isotypes were used to determine the genotype of thymocytes in chimeric mice. In thymic graft chimeras and young bone marrow chimeras, the Fv-1 type of thymocytes rather than thymic stroma determined the amount of ecotropic virus expressed by thymocytes. In older bone marrow chimeras, high amounts of ecotropic virus were expressed by thymocytes independent of Fv-1 type. Attempts to repeat and confirm these results have been temporarily delayed by failure of the AKR.Fv-1(b) mice to breed.